Asynchronous Transfer Mode (ATM)
Overview
The "Broadband Integrated Services Digital Network" or B-ISDN is a new telecommunication technology developed by the telecommunications (carrier) industry for both data transmissions (computer) and telecommunications (telephone). This is conceived as a carrier service to provide high speed communications to end users in an integrated way. The technology selected to deliver the B-ISDN service is called "Asynchronous Transfer Mode" or ATM. The almost universal acceptance of ATM comes from the fact, that:
ATM handles all kinds of communication traffic, such as voice, data, image, video, high quality sound, multimedia and many others, and
ATM can be used in both the LAN (Local Area Network) and the WAN (Wide Area Network) network environments and hence promises seamless inter-working between the two.
Thus ATM is effective in a much wider range of communications environments than any previous technology. However ATM is a compromise.
ATM does not handle voice as efficiently (or as cost effectively) as does an isochronous network.
ATM does does not handle video as easily as isochronous transfer does (although it is a lot more efficient).
ATM certainly does not handle data as effectively or efficiently as a Packet Transfer Mode or Frame Relay system, and
ATM is likely to be problematic in any high error rate environment (such as some slower copper wire connections).
Nevertheless ATM handles all types of traffic adequately and in an integrated way. This means that instead of having a proliferation of many specialised kinds of equipment for different functions it is now possible to have a single type of equipment and network which will do everything.
Technology and Market Trends
Communication technologies have realized considerable progress and many potential applications that were not possible before are now becoming accessible and attractive. One of the driving forces behind ATM development is the need for a communication network architecture that could take advantage of the following changes.
High speed rates can now be sustained with very low bit error rates with the maturing of new transmission media and especially of optical fiber.
The very real user demand for data communication services and for ever faster services has caused carriers to look for an integrated way of supplying these services. Running separate and disparate networks is very expensive.
Silicon chip technology has improved to the point where it can be used to build very fast switching systems.
The general belief that integrated packet (or cell) based switching systems are significantly lower in cost than Time Division Multiplexed (TDM) type systems.
The development (again due to improvement in silicon technology) of much faster and lower-cost computer hardware which makes many new applications possible that were not economically feasible before.
ATM Concepts
The principle key concepts of ATM are as follows:
Cells: all information (voice, image, video, data . . . ) is transported through the network in very short, fixed length (48 data bytes plus a 5-byte header) blocks called "cells". The ATM cell size was determined by the CCITT (now called ITU) as a compromise between voice and data requirements.
Routing: information flow along paths (called "virtual channels") is set up as a series of pointers through the network. The cell header contains an identifier that links the cell to the correct path towards its destination. Cells on a particular virtual channel always follow the same path through the network and are delivered to the destination in the same order in which they are received.
Hardware-Based Switching: ATM is designed so that simple hardware based logic elements may be employed at each node to perform the switching. On a link of 1 Gbps, a new cell arrives and a cell is transmitted every 0.43 microsec.
Adaptation: at the edges of the network, user data frames are broken up into cells. Continuous data streams such as voice and video are segmented into cells. At the destination side of the network the user data frames are reconstructed from the received cells and returned to the end user in the form (data frames etc.) that they were delivered to the network. This adaptation function is considered part of the network but is in a higher layer function, which is called ATM Adaptation Layer (AAL).
Error Control: the ATM cell switching network typically only checks cell headers for errors and simply discards cells in error. The adaptation function AAL is external to the switching network and depends somewhat on the type of traffic but for data traffic it usually checks for errors in data frames received and if one is found then it discards the whole frame. At no time does the ATM network attempt to recover from errors by the re-transmission of information. This function is up to the end user devices and depends on the type of traffic being carried and the adaptation layer being used.
Flow Control: an ATM network may have no internal flow control of any kind. The required processing logic is too complex for flow to be control easily accommodated at the speeds involved. Instead ATM typically has a set of input rate controls that limit the rate of traffic delivered to the network.
Congestion Control: there is only one thing an ATM network can do when a link or node becomes congested. Cells are discarded until the problem has been relieved. Some (lower priority) cells can be marked such that they are the first to be discarded in the case of congestion. Connection end-points are not notified when cells are discarded. It is up to the adaptation function or higher-layer protocols to detect and recover from the loss of cells.
Information about ATM and other high speed networks can be found for example in
"High Speed Networking Technology: An Introductory Survey", June 1993, Document Number GG24-3816-01, IBM International Technical Support Center, Raleigh USA.
"Networking Broadband Services (NBBS): Architecture Tutorial", June 1995, Document Number GG24-4486-00, IBM International Technical Support Center, Raleigh USA.
"Asynchronous Transfer Mode (Broadband ISDN): Technical Overview", June 1994, Document Number GG24-4330-00, IBM International Technical Support Center, Raleigh USA.
Information on the ATM Standards can be found in the International Telecommunication Union (ITU) Recommendations.